Carrier-wave telephony transmitters for the transmission of single-sideband speech signals



March 6, 1962 J. ENSINK ETAL 3,024,313

CARRIER-WAVE TELEPHONY TRANSMITTERS FOR THE TRANSMISSIQN OFSINGLE-SIDEBAND SPEECH SIGNALS Filed Nov. 25, 1955 4 f: 3 5 8 I 6AMPLIFIER 7 r w W W 4 v M a l MIXER Wwgg F'LTER g a'fuo 13 cownsssongr???- FILTER {1 1p 43 4 f PILOT c susmu. GENERATOR gl ll'LAronAMPLIFIER L LIMITER (THRESHOLD RECTIFIER F 1 DEVICE an 5 1152 1.9 :4 :24E OSCILLATOR GENERATOR INVENTORS JOHANNES ENSI NK JAN VERHAGEN AGENTCAR-WAVE TELEPHONY TRANSMITTERS FOR TIE TRANSMISSION OF SINGLE-SIDE-BAND SPEECH SIGNALS Johannes Ensink and Jan Verhagen, Hilversum,Netherlands, assignors, by mesne assignments, to North American PhilipsCompany, Inc., New York, N.Y., a corporation of Delaware Filed Nov. 25,1955, Ser. No. 549,086 Claims priority, application Netherlands Dec. 2,1954 2 Claims. (Cl. 179-155) The invention relates to a carrier wavetelephony transmitter for the transmission of single-sideband speechsignals in which each speech channel is provided with a dynamic controlcircuit in order to produce volume compression.

It is an object of the invention to provide a particularly advantageouscarrier wave telephony transmitter of the kind described in thepreamble, by means of which inter alia a high quality signaltransmission can be realized in practice.

According to the invention, in each speech channel the single-sidebandsignal to be transmitted is supplied together with a pilot signal havinga frequency adjacent the signal frequency band and supplied by a pilotgenerator to the dynamic control circuit which is controlled by acompression voltage derivedfrom a compression voltage rectifier which isfed by the single-sideband speech signal and the pilot signal.

In order to obtain quiet control it has proved of advantage for thesingle-sideband speech signal and the pilot signal to be supplied to thecompression voltage rectifier via a limiter.

In order that the invention may be readily put into practice, oneembodiment thereof will now be described in detail with reference to theaccompanying drawing, in which:

FIG. 1 is a block diagram of a speech channel of a carrier wavetelephony transmitter in accordance with the, invention;

FIG. 2 shows a control characteristic of the compression circuitarrangement used, whilst FIG. 3 is a diagram in greater detail of thespeech channel shown in FIG. 1.

The speech channel shown in FIG. 1 must transmit speech signals whichare situated, for example, in the speech band of from 0.3 kc./s. to 3.2kc./s. For the speech channel concerned a band width of 4 kc./s. isreserved.

In this circuit arrangement the speech signals supplied by a microphone1 are supplied via a filter 2 having a pass band of from 0.3 kc./s. to3.2 kc./s., which filter passes the speech signals, to a single-sidebandmodulator. This modulator comprises a mixer stage 3 which is connectedto a local oscillator 4 of frequency, for example, 30 kc./s. and asingle-sideband filter 5 which passes the upper side band of from 30.3to 33.2 kc./s. This singlesideband signal is supplied afteramplification in a channel amplifier 6 through a channel filter 7 forfurther manipulation to a group modulator which is not shown in thefigure.

In order to obtain volume contrast compression a dynamic control circuit8 is connected between the singlesideband filter 5 and the channelamplifier 6. This dynamic control circuit comprises an adjustabledamping network which is controlled by a compression voltage and isconstituted by rectifier cells. To this dynamic control circuit aresupplied the single-sideband speech signals of from 30.3 to 33.2 kc./s.and also through the singlesideband modulator 3, 4, 5 a pilot signalsupplied by a pilot generator 9, the frequency of which signal issituated adjacent to the frequencies of the signal band. This niteStates Patent 3,024,313 Patented Mar. 6, 1962 level of the pilot signalis arranged to be substantially lower, for example 20 db lower, than thespeech signals at the normal speech level, whilst the frequency of thepilot signal is made to be higher than the speech frequency band, andfor example is 3.7 kc./s. After frequency conversion in thesingle-sideband modulator 3, 4, 5 the pilot signal has a frequency of33.7 kc./s. and is supplied to the dynamic control circuit 8.

In order to generate the compression voltage for the dynamic controlcircuit 8 the arrangement described contains a compression voltagerectifier 10 which is fed by the output voltage of the channel amplifier6 and has an adjusting time constant of, say, 3 msec. and a readjustingtime constant of 30 msec. By rectification of the single-sideband speechsignal, across the output circuit of the rectifier 10 a voltage isproduced which varies with the envelope of the single sideband signal,which voltage follows the envelope of the speech signals with a higherdegree of accuracy than the envelope signal which would have beenobtained by direct rectification of the low frequency speech signals.The output voltage of the rectifier 10 is supplied through a thresholddevice 11 and a direct current amplifier '12 as the compression voltageto the dynamic control circuit 8 which, due to variation in the damping,provides a compression of the singlesideband speech signal supplied toit.

When, in the arrangement described, the level of the speech signals isincreased, this level increase produces a corresponding increase in theoutput voltage of the compression voltage rectifier 10 which is suppliedthrough the threshold device 11 and the direct current amplifier 12 tothe dynamic control circuit 8. Thus, the damping of the dynamic controlcircuit 8 is increased, which results in a damping increasecounteracting the level increase, whilst conversely a decrease in thelevel of the speech signals produces a decrease in the damping of thedynamic control circuit 8. In the dynamic control circuit 8 the pilotsignal is subjected to a corresponding variation in damping andconsequently by its amplitude characterizes the compression of thesingle-side-band speech signal supplied to the dynamic control circuit8.

In the circuit arrangement shown the output signals of the channelamplifier 6 are supplied to the rectifier 10 via a limiter 13 which mustlimit any sudden voltage peaks. The level at which limiting sets in ischosen such that with continuous signals corresponding to loud speech nolimiting occurs, for example in the embodiment shown the limitingvoltage is approximately 1.0 volt higher than the threshold voltage ofthe threshold device 11. This limiter ensures quiet control whilst, inaddition, distortions produced by these voltage peaks are prevented.

The above explanations are based on the condition of operation in whichthe speech level exceeds the pilot signal level. When the speech levelfalls below the pilot level or when no speech signals are produced, thecompression voltage is substantially determined by the constant pilotvoltage which determines the minimum damping of the dynamic controlcircuit 8. Thus, the lower limit of the compression control range isdetermined by the level of the pilot signal or, in other words,adjustment of the pilot level, for example by means of a voltagedivider, permits of adjusting the control range of the compressionarrangement at will.

In this compression arrangement reduction of the control range ensuresthat interference signals, for example room noise, which occur duringspeech intervals are transmitted with restricted amplification only.

FIG. 2 shows the control characteristic of the described compressionarrangement in which in db the pilot level which occurs at the output ofthe dynamic control circuit and the level of the single-sideband speechsignal are plotted as a function of the level of the single-sidebandsignal V which is supplied to the input of the dynamic control circuit.In this control characteristic the pilot level at the input of thedynamic control circuit 8, which for example is lower by 20 db than thenormal speech level, is shown by the point P.

As has been described above, at an input level of the single-sidebandspeech signal which is less than the input level of the pilot signal,the dynamic control circuit 8 exhibits a constant damping, whilst inaddition to damping of the dynamic control circuit 8 increases with thelevel of the said single-sideband signal. In this event in the controlcharacteristic shown the variation of the pilot level is shown by theline a, whilst the line b shows the level variation of thesingle-sideband speech signal. The compression control range is shown inthe figure by PQ and comprises approximately 30 db.

"In the example shown the compression factor is chosen very high, whichis of particular advantage in view of crosstalking between the variouschannels, since as a result level variations between the signalsassociated with the various transmitter channels are reduced to aminimum. Thus, for example, in the compression control range PQ ofapproximately 30 db the level of the single-sideband speech signal whichoccurs at the output of the dynamic control circuit varies only by 0.5db. If required, the remaining level variations may be completelyavoided by the use of a limiter connected between the dynamic controlcircuit and the end of the channel.

In spite of the very high compression factor the quality of transmissionproves to be excellent and, as has been found experimentally, materiallybetter than the trans mission quality with the use of direct compressionof the low frequency speech signals. At the receiver end an expansioncontrol dependent upon the amplitude of the pilot which characterizesthe level variations of the initial signal permits of recovering theinitial speech signal substantially completely. The use of the pilotsignal has the additional advantage that even high variations in dampingof, for example, from 10 to 20 db in the transmitting channel do notaffect the quality of transmission, whilst in addition the rectifiercells of the dynamic control circuit need not satisfy exactingrequirements.

It should be noted here that in the compression arrangement shown, theback control may be replaced forward control, for example by connectingthe compression voltage rectifier to the output of the single-sidebandfilter 5.

FIG. 3 shows in detail the transmitting channel shown in block-schematicform in FIG. 1.

In the arrangement shown in FIG. 3 the speech signals supplied by amicrophone 14- are supplied via a filter 15 having a pass-band of from0.3-3.2 kc./s. to a single-sideband modulator in the form of a ringmodulator which comprises an input transformer 16 and an outputtransformer 17, rectifier cells 18, 19, 20 21 being provided between theends of the secondary winding of the input transformer 16 and theprimary winding of the output transformer 17. The input transformer '16has a second primary winding 22 to which via a series resistor 23 apilot signal of 3.7 kc./s. supplied by a pilot generator 24 is supplied,whilst a local carrier wave oscillator 25 of kc./ s. is connected to thecentre taps on the secondary winding of the input transformer 16 and theprimary winding of the output transformer 17. The output signals of thering modulator are supplied via a single-sideband filter 26 which passesthe upper side band of from 30.3- 33.2 kc./ s. and the converted pilotsignal of 33.7 kc./ s. and via a dynamic control circuit to a channelamplifier 27 which through an output transformer 28 is connected to achannel filter 29. Thus, at the output terminals 30, 31 of the channelfilter the single-sideband speech signal and the pilot signal areproduced.

In the embodiment shown the dynamic control circuit is provided with aninput transformer 32 and an output transformer 33, whilst between theends of the secondary winding of the input transformer 32 and theprimary Winding of the output transformer 33 a damping network isconnected comprising the cascade connection of two voltage dividerswhich comprise resistors 34, 35 and 36, 37 respectively and rectifiercells, for example germanium cells 3 8, 39 and 40, 41 respectively whichare connected between said resistors, whilst the compression voltagelead 42 is connected to the junctions of the germanium cells 38, 39 and40, 41 respectively. The germanium cells 38, 39 and 40, 41 respectivelyact as variable resistors which for compression control are controlledby the compression voltage.

In order to generate the compression voltage the output transformer 28has a coil 43 connected to it which through a limiter arrangementcomprising a series resistor 44' and a biassed rectifier cell 44 forlimiting sudden voltage surges is connected to a compression voltagerectifier 45 provided with an output impedance comprising a capacitor 47which is bridged by a resistor 46, the charge time constant and thedischarge time constant (adjustment and readjustment constantrespectively) being, for example, 3 and 30 msec. respectively. Theoutput voltage of the rectifier 45 via a direct current amplifier 49,which by means of a grid voltage battery 48 connected in series with therectifier output impedance is connected in class C and consequently alsoacts as a threshold device, is supplied as compression voltage to thedynamic control circuit, which compression voltage, as has beendescribed with reference to FIG. 1, ensures a compression of thesingle-sideband speech signal in the dynamic control circuit. Thus, thecompression control direct voltage supplied to the dynamic controlcircuit cannot penetrate into the compression control circuit throughthe channel amplifier which passes only higher frequencies, so thatinstabilities are prevented.

It should be noted here that it is not absolutely necessary to useexpansion control at the receiver end, for the intelligibility andrecognizability of voices is very satisfactory without the provision ofan expansion control at the receiver end.

What is claimed is:

1. A transmitter comprising a plurality of signal channels, each signalcomprising a source of a single-sideband signal lying in a frequencyband, a source of a pilot signal having a frequency adjacent to saidfrequency band, a dynamic compression control circuit having input andoutput circuits for signals to be controlled and having a control signalinput terminal, means connected to apply both of said single-sidebandand said pilot signals to said input circuit, means including arectifier for producing a control signal from both of saidsingle-sideband and said pilot signals, means connected to apply saidcontrol signal to said control signal input terminal, a limiter, andmeans connected to apply said single-sideband and said pilot signals tosaid rectifier through said limiter.

2. A transmitter for the transmission of a plurality of single-sidebandspeech signals originating from a plurality of separate speech channelseach of said channels comprising a source of a single-sideband signal, asource of a pilot signal having a frequency adjacent saidsingle-sideband signal, dynamic compression means having input andoutput circuits for signals to be controlled and a control signal inputterminal, means connected to apply said single sideband signal and pilotsignal to said input circuit, means producing a control signal from saidsingle-sideband signal and pilot signal comprising limiter means andrectifier means, and means applying said control signal to said inputterminal, and means combining the signals from the output circuits ofthe compression means of each of said channels.

References Cited in the file of this patent UNITED STATES PATENTS2,006,989 Frantz et a] July 2, 1935 2,007,416 Atfel July 9, 19352,757,239 Patton July 31, 1956 2,768,352 Von Sivers et al Oct. 23, 1956

